Sulfate Chemistry for High-Voltage Insertion Materials: Synthetic, Structural and Electrochemical Insights

Rechargeable batteries based on Li and Na ions have been growing leaps and bounds since their inception in the 1970s. They enjoy significant attention from both the fundamental science point of view and practical applications ranging from portable electronics to hybrid vehicles and grid storage. The steady demand for building better batteries calls for discovery, optimisation and implementation of novel positive insertion (cathode) materials. In this quest, chemists have tried to unravel many future cathode materials by taking into consideration their eco-friendly synthesis, material/process economy, high energy density, safety, easy handling and sustainability. Interestingly, sulfate-based cathodes offer a good combination of sustainable syntheses and high energy density owing to their high-voltage operation, stemming from electronegative SO42- units. This review delivers a sneak peak at the recent advances in the discovery and development of sulfate-containing cathode materials by focusing on their synthesis, crystal structure and electrochemical performance. Several family of cathodes are independently discussed. They are 1) fluorosulfates AMSO(4)F], 2) bihydrated fluorosulfates AMSO(4)F2H(2)O], 3) hydroxysulfate AMSO(4)OH], 4) bisulfates A(2)M(SO4)(2)], 5) hydrated bisulfates A(2)M(SO4)(2)nH(2)O], 6) oxysulfates Fe-2(SO4)(2)O] and 7) polysulfates A(2)M(2)(SO4)(3)]. A comparative study of these sulfate-based cathodes has been provided to offer an outlook on the future development of high-voltage polyanionic cathode materials for next-generation batteries.

Dendron conjugation to graphene oxide using click chemistry for efficient gene delivery

Owing to its large surface area and rapid cellular uptake, graphene oxide (GO) is emerging as an attractive candidate material for delivery of drugs and genes. The inherent sp(2) pi-pi interaction of GO helps to carry drugs and single stranded RNA (ssRNA) but there is no such interaction with double stranded DNA (dsDNA). In this work, a polyamidoamine (PAMAM) dendron was conjugated with nano GO (nGO) through ``click'' chemistry to improve the DNA complexation capability of GO as well as its transfection efficiency. The DNA complexation capability of GO was significantly enhanced after dendronization of GO yielding spherical nanosized (250-350 nm) particles of the dendronized GO (DGO)/pDNA complex with a positive zeta potential. The transfection efficiency of GO dramatically increased after conjugation of the PAMAM dendron. Transfection efficiency of 51% in HeLa cells with cell viability of 80% was observed. The transfection efficiency was significantly higher than that of polyethyleneimine 25 kDa (27% efficiency) and also surpassed that of lipofectamine 2000 (47% efficiency). The uptake of the DGO/pDNA complex by the caveolae mediated endocytosis pathway may significantly contribute to the high transfection efficiency. Thus, dendronized GO is shown to be an efficient gene carrier with minimal toxicity and is a promising candidate for use as a nonviral carrier for gene therapy.

Remarkable Effect of Chalcogen Substitution on an Enzyme Mimetic for Deiodination of Thyroid Hormones

Iodothyronine deiodinases are selenoenzymes which regulate the thyroid hormone homeostasis by catalyzing the regioselective deiodination of thyroxine (T4). Synthetic deiodinase mimetics are important not only to understand the mechanism of enzyme catalysis, but also to develop therapeutic agents as abnormal thyroid hormone levels have implications in different diseases, such as hypoxia, myocardial infarction, critical illness, neuronal ischemia, tissue injury, and cancer. Described herein is that the replacement of sulfur/selenium atoms in a series of deiodinase mimetics by tellurium remarkably alters the reactivity as well as regioselectivity toward T4. The tellurium compounds reported in this paper represent the first examples of deiodinase mimetics which mediate sequential deiodination of T4 to produce all the hormone derivatives including T0 under physiologically relevant conditions.

Deciphering complex patterns of class-I HLA-peptide cross-reactivity via hierarchical grouping

T-cell responses in humans are initiated by the binding of a peptide antigen to a human leukocyte antigen (HLA) molecule. The peptide-HLA complex then recruits an appropriate T cell, leading to cell-mediated immunity. More than 2000 HLA class-I alleles are known in humans, and they vary only in their peptide-binding grooves. The polymorphism they exhibit enables them to bind a wide range of peptide antigens from diverse sources. HLA molecules and peptides present a complex molecular recognition pattern, as many peptides bind to a given allele and a given peptide can be recognized by many alleles. A powerful grouping scheme that not only provides an insightful classification, but is also capable of dissecting the physicochemical basis of recognition specificity is necessary to address this complexity. We present a hierarchical classification of 2010 class-I alleles by using a systematic divisive clustering method. All-pair distances of alleles were obtained by comparing binding pockets in the structural models. By varying the similarity thresholds, a multilevel classification was obtained, with 7 supergroups, each further subclassifying to yield 72 groups. An independent clustering performed based only on similarities in their epitope pools correlated highly with pocket-based clustering. Physicochemical feature combinations that best explain the basis of clustering are identified. Mutual information calculated for the set of peptide ligands enables identification of binding site residues contributing to peptide specificity. The grouping of HLA molecules achieved here will be useful for rational vaccine design, understanding disease susceptibilities and predicting risk of organ transplants.

Contrasting Electronic Requirements for C-H Binding and C-H Activation in d(6) Half-Sandwich Complexes of Rhenium and Tungsten

A computational study of the interaction half-sandwich metal fragments (metal=Re/W, electron count=d(6)), containing linear nitrosyl (NO+), carbon monoxide (CO), trifluorophosphine (PF3), N-heterocyclic carbene (NHC) ligands with alkanes are conducted using density functional theory employing the hybrid meta-GGA functional (M06). Electron deficiency on the metal increases with the ligand in the order NHC < CO < PF3 < NO+. Electron-withdrawing ligands like NO+ lead to more stable alkane complexes than NHC, a strong electron donor. Energy decomposition analysis shows that stabilization is due to orbital interaction involving charge transfer from the alkane to the metal. Reactivity and dynamics of the alkane fragment are facilitated by electron donors on the metal. These results match most of the experimental results known for CO and PF3 complexes. The study suggests activation of alkane in metal complexes to be facile with strong donor ligands like NHC. (C) 2015 Wiley Periodicals, Inc.

Structure Elucidation and Characterization of Different Thyroxine Polymorphs

Thyroid hormones regulate almost every process in the body, including body temperature, growth, and heart rate. They influence carbohydrate metabolism, protein synthesis and breakdown, and cardiovascular, renal, and brain function. Two new polymorphs of synthetic L-thyroxine (T4) are reported and the effect of polymorphism on the solubility of this important hormone is shown. Conformational changes were also discovered to have a remarkable effect on the strength of halogen bonding and the reactivity of the CI bonds, which could have a significant effect on the hormone activity.

TURBULENCE-TRANSPORT-CHEMISTRY INTERACTION IN STATISTICALLY PLANAR PREMIXED FLAMES AND IGNITION KERNELS IN NEAR ISOTROPIC TURBULENCE

Turbulence-transport-chemistry interaction plays a crucial role on the flame surface geometry, local and global reactionrates, and therefore, on the propagation and extinction characteristics of intensely turbulent, premixed flames encountered in LPP gas-turbine combustors. The aim of the present work is to understand these interaction effects on the flame surface annihilation and extinction of lean premixed flames, interacting with near isotropic turbulence. As an example case, lean premixed H-2-air mixture is considered so as to enable inclusion of detailed chemistry effects in Direct Numerical Simulations (DNS). The work is carried out in two phases namely, statistically planar flames and ignition kernel, both interacting with near isotropic turbulence, using the recently proposed Flame Particle Tracking (FPT) technique. Flame particles are surface points residing and commoving with an iso-scalar surface within a premixed flame. Tracking flame particles allows us to study the evolution of propagating surface locations uniquely identified with time. In this work, using DNS and FPT we study the flame speed, reaction rate and transport histories of such flame particles residing on iso-scalar surfaces. An analytical expression for the local displacement flame speed (SO is derived, and the contribution of transport and chemistry on the displacement flame speed is identified. An examination of the results of the planar case leads to a conclusion that the cause of variation in S-d may be attributed to the effects of turbulent transport and heat release rate. In the second phase of this work, the sustenance of an ignition kernel is examined in light of the S-curve. A newly proposed Damkohler number accounting for local turbulent transport and reaction rates is found to explain either the sustenance or otherwise propagation of flame kernels in near isotropic turbulence.

Insights into the catalytic mechanism of synthetic glutathione peroxidase mimetics

Glutathione Peroxidase (GPx) is a key selenoenzyme that protects biomolecules from oxidative damage. Extensive research has been carried out to design and synthesize small organoselenium compounds as functional mimics of GPx. While the catalytic mechanism of the native enzyme itself is poorly understood, the synthetic mimics follow different catalytic pathways depending upon the structures and reactivities of various intermediates formed in the catalytic cycle. The steric as well as electronic environments around the selenium atom not only modulate the reactivity of these synthetic mimics towards peroxides and thiols, but also the catalytic mechanisms. The catalytic cycle of small GPx mimics is also dependent on the nature of peroxides and thiols used in the study. In this review, we discuss how the catalytic mechanism varies with the substituents attached to the selenium atom.

Crystal chemistry and photomechanical behavior of 3,4-dimethoxycinnamic acid: correlation between maximum yield in the solid-state topochemical reaction and cooperative molecular motion

A new monoclinic polymorph, form II (P2(1)/c, Z = 4), has been isolated for 3,4-dimethoxycinnamic acid (DMCA). Its solid-state 2 + 2 photoreaction to the corresponding alpha-truxillic acid is different from that of the first polymorph, the triclinic form I (P (1) over bar, Z = 4) that was reported in 1984. The crystal structures of the two forms are rather different. The two polymorphs also exhibit different photomechanical properties. Form I exhibits photosalient behavior but this effect is absent in form II. These properties can be explained on the basis of the crystal packing in the two forms. The nanoindentation technique is used to shed further insights into these structure-property relationships. A faster photoreaction in form I and a higher yield in form II are rationalized on the basis of the mechanical properties of the individual crystal forms. It is suggested that both Schmidt-type and Kaupp-type topochemistry are applicable for the solid-state trans-cinnamic acid photodimerization reaction. Form I of DMCA is more plastic and seems to react under Kaupp-type conditions with maximum molecular movements. Form II is more brittle, and its interlocked structure seems to favor Schmidt-type topochemistry with minimum molecular movement.

A flame particle tracking analysis of turbulence-chemistry interaction in hydrogen-air premixed flames

Interactions of turbulence, molecular transport, and energy transport, coupled with chemistry play a crucial role in the evolution of flame surface geometry, propagation, annihilation, and local extinction/re-ignition characteristics of intensely turbulent premixed flames. This study seeks to understand how these interactions affect flame surface annihilation of lean hydrogen-air premixed turbulent flames. Direct numerical simulations (DNSs) are conducted at different parametric conditions with a detailed reaction mechanism and transport properties for hydrogen-air flames. Flame particle tracking (FPT) technique is used to follow specific flame surface segments. An analytical expression for the local displacement flame speed (S-d) of a temperature isosurface is considered, and the contributions of transport, chemistry, and kinematics on the displacement flame speed at different turbulence-flame interaction conditions are identified. In general, the displacement flame speed for the flame particles is found to increase with time for all conditions considered. This is because, eventually all flame surfaces and their resident flame particles approach annihilation by reactant island formation at the end of stretching and folding processes induced by turbulence. Statistics of principal curvature evolving in time, obtained using FPT, suggest that these islands are ellipsoidal on average enclosing fresh reactants. Further examinations show that the increase in S-d is caused by the increased negative curvature of the flame surface and eventual homogenization of temperature gradients as these reactant islands shrink due to flame propagation and turbulent mixing. Finally, the evolution of the normalized, averaged, displacement flame speed vs. stretch Karlovitz number are found to collapse on a narrow band, suggesting that a unified description of flame speed dependence on stretch rate may be possible in the Lagrangian description. (C) 2015 The Combustion Institute. Published by Elsevier Inc. All rights reserved.

Inter-comparison and performance evaluation of chemistry transport models over Indian region

Aerosol loading over the South Asian region has the potential to affect the monsoon rainfall, Himalayan glaciers and regional air-quality, with implications for the billions in this region. While field campaigns and network observations provide primary data, they tend to be location/season specific. Numerical models are useful to regionalize such location-specific data. Studies have shown that numerical models underestimate the aerosol scenario over the Indian region, mainly due to shortcomings related to meteorology and the emission inventories used. In this context, we have evaluated the performance of two such chemistry-transport models: WRF-Chem and SPRINTARS over an India-centric domain. The models differ in many aspects including physical domain, horizontal resolution, meteorological forcing and so on etc. Despite these differences, both the models simulated similar spatial patterns of Black Carbon (BC) mass concentration, (with a spatial correlation of 0.9 with each other), and a reasonable estimates of its concentration, though both of them under-estimated vis-a-vis the observations. While the emissions are lower (higher) in SPRINTARS (WRF-Chem), overestimation of wind parameters in WRF-Chem caused the concentration to be similar in both models. Additionally, we quantified the under-estimations of anthropogenic BC emissions in the inventories used these two models and three other widely used emission inventories. Our analysis indicates that all these emission inventories underestimate the emissions of BC over India by a factor that ranges from 1.5 to 2.9. We have also studied the model simulations of aerosol optical depth over the Indian region. The models differ significantly in simulations of AOD, with WRF-Chem having a better agreement with satellite observations of AOD as far as the spatial pattern is concerned. It is important to note that in addition to BC, dust can also contribute significantly to AOD. The models differ in simulations of the spatial pattern of mineral dust over the Indian region. We find that both meteorological forcing and emission formulation contribute to these differences. Since AOD is column integrated parameter, description of vertical profiles in both models, especially since elevated aerosol layers are often observed over Indian region, could be also a contributing factor. Additionally, differences in the prescription of the optical properties of BC between the models appear to affect the AOD simulations. We also compared simulation of sea-salt concentration in the two models and found that WRF-Chem underestimated its concentration vis-a-vis SPRINTARS. The differences in near-surface oceanic wind speeds appear to be the main source of this difference. In-spite of these differences, we note that there are similarities in their simulation of spatial patterns of various aerosol species (with each other and with observations) and hence models could be valuable tools for aerosol-related studies over the Indian region. Better estimation of emission inventories could improve aerosol-related simulations. (C) 2015 Elsevier Ltd. All rights reserved.

Triplet excited electronic state switching induced by hydrogen bonding: A transient absorption spectroscopy and time-dependent DFT study

The solvent plays a decisive role in the photochemistry and photophysics of aromatic ketones. Xanthone (XT) is one such aromatic ketone and its triplet-triplet (T-T) absorption spectra show intriguing solvatochromic behavior. Also, the reactivity of XT towards H-atom abstraction shows an unprecedented decrease in protic solvents relative to aprotic solvents. Therefore, a comprehensive solvatochromic analysis of the triplet-triplet absorption spectra of XT was carried out in conjunction with time dependent density functional theory using the ad hoc explicit solvent model approach. A detailed solvatochromic analysis of the T-T absorption bands of XT suggests that the hydrogen bonding interactions are different in the corresponding triplet excited states. Furthermore, the contributions of non-specific and hydrogen bonding interactions towards differential solvation of the triplet states in protic solvents were found to be of equal magnitude. The frontier molecular orbital and electron density difference analysis of the T-1 and T-2 states of XT indicates that the charge redistribution in these states leads to intermolecular hydrogen bond strengthening and weakening, respectively, relative to the S-0 state. This is further supported by the vertical excitation energy calculations of the XT-methanol supra-molecular complex. The intermolecular hydrogen bonding potential energy curves obtained for this complex in the S-0, T-1, and T-2 states support the model. In summary, we propose that the different hydrogen bonding mechanisms exhibited by the two lowest triplet excited states of XT result in a decreasing role of the n pi* triplet state, and are thus responsible for its reduced reactivity towards H-atom abstraction in protic solvents. (C) 2016 AIP Publishing LLC.